Active distributed signal injector

ABSTRACT

A circuit that injects an auxiliary signal into an existing signal path, thereby adding it to an existing main signal, while minimizing attenuation, distortion, or other perturbation to the main signal. The present invention provides for a active distributed signal injector that includes two transmission lines. An input port is coupled to the first transmission line for receiving the main signal, and a main output port is coupled to an output end of the first transmission line for outputting the main signal. The second transmission line is terminated at an output end and has an injected signal input port at an input end of the second transmission line. A plurality of high impedance amplifiers bridge between the transmission lines, and a plurality of inductors are coupled between each of the high impedance amplifiers. A plurality of transmission lines may be optionally employed in the second transmission line. Optionally, the termination used in the second transmission line may be replaced by a second output port that is used to output the injected signal.

BACKGROUND

The present invention relates generally to signal injectors, and moreparticularly, to an active distributed signal injector for use with highfrequency systems, whose components are interconnected by low lossconstant impedance transmission lines.

In many electronic systems, the need arises to inject an auxiliarysignal into an existing signal path, while minimizing attenuation,distortion, or other perturbation of the through signal. Some examplesof such systems include interference or distortion cancellationcircuits, interference suppressors, feedforward amplifiers, andequipment for testing or monitoring, electronic components and systems.

One prior art signal injection circuit uses a low-loss passive coupler.A loose coupling, typically -20 dB, is used to minimize degradation tothe main signal caused by attenuation or distortion in phase andamplitude response. Unfortunately the coupling loss suffered by theinjected signal has to be made up by an additional amplifier, most ofwhose output is wasted in the termination at the output port. Thus, acompromise has to be made between loss to the injected signal and themain signal, but attenuation to either requires additionalamplification, which adds its own penalty in signal degradation, power,cost, weight, and space.

Another prior art signal injection circuit uses a high impedanceamplifying device, such as a field effect transistor (FET). This circuitavoids the penalty of wasting power in a termination, but, since thecircuit has no directivity, half the power is wasted in the sourceimpedance of the main input. This unwanted signal component, whichtravels in the wrong direction, can also cause other difficulties, suchas harmful feedback and reflections. Also, lumping all the amplifyingfunction in one large device, makes impedance matching difficult.

Distributed amplifiers are similar to the present invention. Distributedamplifiers are disclosed in a number of U.S. patents. For example, U.S.Pat. No. 4,359,695 entitled "Electronic amplification systems" disclosesa distributed amplifier system allowing easier correction of amplifiererrors. This invention provides an electronic amplification systemincorporating a distributed amplifier, which system comprises comparisonmeans for comparing the shape of the input control signal with the shapeof the final output signal, and for providing a difference signalindicative of any amplifier-induced distortion, correction signalgenerating means for forming in dependence upon the difference signal acorrection signal, and combiner means for combining the correctionsignal with the final output signal so as substantially to cancel theamplifier-induced distortion.

U.S. Pat. No. 4,845,440 entitled "Distributed amplifier circuits"discloses a wide band amplifier for operation at very high frequenciesthat comprises a MESFET distributed amplifier having a gate and a draintransmission line, a first hybrid circuit to apply a first and secondinput signal to opposite ends of the gate transmission line, and asecond hybrid circuit connected to opposite ends of the draintransmission line to receive and combine first and second output signalsfrom the drain transmission line to provide an amplified output signal.The use of two input signals traveling in opposite directions along thegate transmission line increases the gain which can be achieved in thedistributed amplifier and reduces the noise component of the outputsignal. This circuit is useful for enhancing the performance of adistributed amplifier containing only a few MESFETs.

U.S. Pat. No. 5,196,805 entitled "Distributed differential amplifierarrangement discloses an N-stage differential distributed amplifierarrangement. The differential distributed amplifier arrangement includesa parallel connection of N-differential amplifiers. The inputs to theamplifiers are delayed so that the same input is received by eachamplifier in sequence at a slightly later time than the precedingamplifier. The outputs of each amplifier are also delayed so that theoutput of the previous amplifier is added to the output of the nextsequential amplifier. Thus, the output is an amplified version of theinput. By appropriate grounding of inputs or outputs the differentialdistributed amplifier arrangement may convert from balanced signals tosingle-ended signals, from single-ended signals to balanced signals orfrom two inputs to two outputs.

U.S. Pat. No. 4,797,628 entitled "Distributed push-pull amplifier"discloses a modified distributed amplifier that is capable of providingpush-pull operation without the loading losses of conventional push-pullcombining. The modified distributed amplifier comprises a distributedamplifier configuration and with signal inverting means, such as a widebandwidth transmission line transformer, interconnected into both theinput and output lines. The signal inverting means are placed at theelectrical centers of the lines, but may be placed at any positions inthe individual lines to produce optimum performance to specificapplications. The separate segments of the distributed amplifierseparated by the signal inverting means operate in opposed phase but thesignals output to the load add in phase thus providing push-pulloperation. Since only one reverse terminating resistor is required, thepower normally lost due to the loading by the companion amplifier of aconventional push-pull combined distributed amplifier system is insteaddelivered to the output load, the use of this type of amplifierpreserves the advantages of the distributed amplifier configurationwhile providing the superior performance of the push-pull configurationbut without the losses normally incurred with conventional push-pullcombining.

U.S. Pat. No. 5,166,640 entitled "Two dimensional distributed amplifierhaving multiple phase shifted outputs" discloses a two dimensionaldistributed amplifier phase shifter having a distributed referenceamplifier circuit for generating a reference signal. The referenceamplifier circuit has its input connected to one end of a plurality ofserially connected microstrip transmission lines. The phase shifter hasa plurality of phase shifted amplifier circuits, one associated witheach of the microstrip transmission lines. Each phase shifted amplifiercircuit has an input connected to an end of its associated microstriptransmission line which is opposite the reference amplifier circuit andgenerates an output signal, phase shifted from the reference signal orthe output signal of an adjacent phase shifted amplifier by apredetermined phase angle.

U.S. Pat. No. 5,177,381 entitled "Distributed logarithmic amplifier andmethod" discloses a logarithmic amplifier with amplifier stages havingan input transmission line and first and second output transmissionlines. The input and output transmission lines are coupled by multipleamplifier elements distributed along the transmission lines. One outputtransmission line forms a high gain low compression path and the other alow gain high compression path. The output transmission lines of eachstage are coupled to a combiner from whence the logarithmicallyamplified output signal is obtained. The logarithmic amplifier stagesare readily constructed in MMIC form and multiple stages may be easilycascaded to provide a very large dynamic range.

U.S. Pat. No. 4,752,746 entitled "Wideband microwave matrix amplifier"discloses a microwave amplifier that multiplicatively and additivelyamplifies microwave frequency signals. The matrix amplifier is adistributed amplifier with two or more tiers (rows) of transistors. Eachtier has a plurality of transistors that additively amplify the signalentering that row of the amplifier, and each row multiplicativelyamplifies the output of the previous row. The gates of the transistorsin each row are sequentially coupled to an input transmission linehaving a series of transmission elements. The outputs of all thetransistors from each row are sequentially coupled to the inputtransmission line of the next tier, except that the outputs of the lasttier are coupled to an output transmission line for transmitting theoutput of the amplifier to an output node. Each transmission lines hasat least one line termination at one of its ends for absorbing signalsincident on that end of the transmission line, and biasing means for DCbiasing the transmission line at a corresponding voltage potential.

U.S. Pat. No. 5,070,304 entitled "Distributed combiner power amplifierand method" discloses a distributed amplifier usable in the gigahertzfrequency range for radar and electronic warfare applications. In theamplifier stage, a combination of optimum amplifier stage loading, areflection canceling output network, and an amplifier capacitance hidinginput arrangement are employed.

Other U.S. patents relate to feedforward amplifiers. These include, forexample, U.S. Pat. No. 4,359,695 entitled "Electronic amplificationsystems" cited above, and U.S. Pat. No. 4,359,696 entitled "Amplifiers"which discloses a feedforward amplifier that compensates for distortionsintroduced by a main amplifying device by comparing the amplified signalwith the original unamplified signal and producing a correction signal,which is related to the difference. The correction signal is combinedwith the main amplified signal at a combiner. The combiner takes theform of a tetrode or pentode valve which acts as a current source andwhich is designed so as to constitute part of an output transmissionline having a predetermined characteristic impedance. By this means thecombiner does not adversely affect the main amplified signal and thecapacitance inevitably associated with a tetrode or pentode valve isutilized as part of the reactance of the transmission line and does notadversely affect the frequency response of the amplifier.

U.S. Pat. No. 4,885,551 entitled "Feed forward linear amplifier"discloses a feed forward circuit receives an input signal having atleast one carrier therein in a prescribed frequency range. The inputsignal is applied to a first circuit path having a power amplifier thatproduces an output signal with a distortion component and to a secondcircuit path that delays the input signal without distortion. The outputsignal of the first circuit path is combined with the output signal ofthe second circuit path to form a signal representative of thedistortion component of the first circuit path output signal and thedistortion component representative signal is subtracted from the outputsignal of the first circuit path to cancel the distortion componenttherein. The prescribed frequency range of the first circuit path outputis scanned to detect a carrier signal and the amplitude and phase of thesignal in the first circuit path is modified to minimize the magnitudeof the detected carrier in the distortion representative signal.

While these last-cited U.S. patents discuss feedforward amplifiers, nonefeature distributed amplifiers with two input signals, one of which isamplified using active devices, while the other is passed throughwithout active amplification. Active amplification is prone to introducedistortion. The present invention could form part of a feedforwardamplifier, but it is not in itself a feedforward amplifier.

Accordingly, it is an objective of the present invention to provide anactive distributed signal injector for use with high frequency systems,whose components are interconnected by low loss constant impedancetransmission lines.

SUMMARY OF THE INVENTION

To meet the above and other objectives, the present invention is a newtype of signal processing element, referred to as an active distributedsignal injector. The active distributed signal injector injects anauxiliary signal into an existing signal path, thereby adding it to anexisting main signal, while minimizing attenuation, distortion, or otherperturbation to the main signal. The active distributed signal injectorincludes first and second transmission lines, and an input port coupledto the first transmission line for receiving the main signal by way ofan input transmission line. An output port is coupled to an output endof the first transmission line for outputting the main signal by way ofan output transmission line. The second transmission line is typicallyterminated at an output end and has an injected signal input port at aninput end for receiving the injected signal. Optionally, the outputtermination may be replaced by a second output port. The first andsecond transmission lines are bridged by a plurality of high impedanceamplifying elements, such as FET devices, that transfer the main signalfrom the input to the output line, amplifying it in the process. Aplurality of inductors are coupled between each of the high impedanceamplifiers. A plurality of impedance transmission lines may beoptionally coupled between respective ones of the inductors disposedbetween the high impedance amplifiers of the second transmission line.

The present invention is based on the principle of distributedamplification, which makes it particularly applicable to high frequencysystems, whose components are interconnected by low loss constantimpedance transmission lines. The present invention is similar to adistributed amplifier but the termination of the output transmissionline is replaced with an additional input port for receiving the mainsignal.

The present invention minimizes attenuation, distortion, or otherperturbations to the main signal, and does not cancel or suppressdistortion. The prior art references cited above are generally unrelatedto the present invention. Some of the references are or employdistributed amplifiers, but none feature distributed amplifiers with twooutput signals, one of which is amplified using active devices, andwherein the other is passed through the circuit without activeamplification. Active amplification is prone to introduce distortion.

The present invention achieves directivity and low loss simultaneously.The present invention provides for minimal perturbation of the mainsignal, resulting in low distortion in phase and amplitude response,resulting in preservation of bandwidth, low dispersion, low loss, andlower loss to the injected signal. Therefore, less amplification of theinjected signal is needed, and less signal degradation, power, weight,and space, and less cost is required, and it is easily implemented inMMIC form. The signal injection is directional and thus there is lowerVSWR and less interaction and interference between signals.

The present invention may be employed with interference or distortioncancellation circuits such as adaptive filters, adaptive equalizers suchas adaptive baseband equalizers and transversal filter equalizers at IF,interference suppressers such as adaptive cross polarizationinterference cancelers, and adaptive phased arrays, feedforwardamplifiers, and equipment for testing or monitoring, electroniccomponents and systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 shows prior art signal injection using a passive coupler;

FIG. 2 shows prior art signal injection using a high impedance device;

FIG. 3 shows prior art distributed amplifier; and

FIG. 4 shows an active distributed signal injector in accordance withthe present invention.

DETAILED DESCRIPTION

Referring to the drawing figures, FIG. 1 shows a prior art signalinjecting circuit using a passive coupler. Loose coupling, typically -20dB, is used to minimize degradation to the main signal caused byattenuation or distortion in phase and amplitude response. Unfortunatelythe coupling loss suffered by the injected signal has to be made up byan additional amplifier, most of whose output is wasted in thetermination at the output port. Thus, a compromise must be made betweenloss to the injected signal and the main signal, but attenuation toeither requires additional amplification, which adds its own penalty insignal degradation, power, cost, weight, and space.

FIG. 2 shows a prior art signal injecting circuit using a high impedancedevice, such as a field effect transistor (FET). This circuit avoids thepenalty of wasting power in a termination, but, since the circuit has nodirectivity, half the power is wasted in the source impedance of themain input. This unwanted signal component, which travels in the wrongdirection, could also cause other difficulties. Also, lumping all theamplifying function in one large device, makes impedance matchingdifficult.

FIG. 3 shows a prior art distributed amplifier that is somewhat similarto the present invention. The prior art distributed amplifier shown inFIG. 3 omits components for DC bias and other ancillary circuitry. Thedistributed amplifier employs two terminated transmission lines, aninput line and an output line, bridged by high impedance amplifiers.These amplifiers, shown as Flit devices, transfer the signal from theinput line to the output line, amplifying it in the process. Thetransmission lines are implemented in lumped form with the amplifiersproviding shunt capacitance. In the distributed amplifier of FIG. 3, theinput transmission line is terminated at its output and the outputtransmission line is terminated at its input.

FIG. 4 shows a preferred embodiment of an active distributed signalinjector 10 in accordance with the present invention, omittingcomponents for DC bias and other ancillary circuitry. The activedistributed signal injector 10 is based on the principle of distributedamplification, which makes it particularly applicable to high frequencysystems.

The active distributed signal injector 10 comprises first and secondtransmission lines 11, 12. A first input port 13 is coupled to a firstend of the first transmission line 11 that comprises an input line 21. Amain signal output port 14 is coupled to a second end of the firsttransmission line 11 that comprises an output line 22. An injectedsignal input port 15 is coupled to a first end of the secondtransmission line 12 by way of a second input line 23. A termination 16is coupled to a second end of the second transmission line 12.Optionally, the termination 16 may be replaced by a second output port14a that is used to output the injected signal.

The first and is second transmission lines 11, 12 are bridged by aplurality of high impedance amplifiers 17. Each of the high impedanceamplifiers 17 are coupled together using series inductors 18. The seriesinductors 18 employed in the second transmission line 12 may beoptionally interconnected by conventional transmission lines 19. Ingeneral both of the first and second transmission lines 11, 12 compriselow loss constant impedance transmission lines 11, 12. The inductanceprovided by the plurality of inductors 18 is taken into account indesigning the transmission lines 11, 12.

The active distributed signal injector 10 is similar to the prior artdistributed amplifier shown in FIG. 3. The differences are that thetermination of the input transmission line of the distributed amplifieris replaced by the main signal port 13 for the main signal, while theinjected signal is input to the primary input of the distributedamplifier. Also care must be used so that the main input transmissionline 21 is low loss, low VSWR, and non-dispersive. This effects thedesign tradeoffs for the amplifying device 17 and the other components.The active distributed signal injector 10 of the present invention maybe easily implemented as microwave monolithic integrated circuits.

The active distributed signal injector 10 injects the auxiliary signal,while minimizing attenuation, distortion, or other perturbation to boththe injected (auxiliary) signal and the main signal. The presentinvention minimizes attenuation, distortion, or other perturbations tothe injected and main signals, and does not cancel or suppressdistortion.

The active distributed signal injector 10 provides for low perturbationof the main signal, resulting in low distortion in phase and amplituderesponse, which preserves bandwidth, and provides for low dispersion,low loss, and lower loss to the main signal. Therefore, lessamplification of the sampled signal is needed, and less signaldegradation, power, weight, and space, and less cost is required, and itis easily implemented in MMIC form. The active distributed signalinjector 10 may be employed with interference or distortion cancellationcircuits such as adaptive filters, adaptive equalizers such as adaptivebaseband equalizers and transversal filter equalizers at interferencesuppressers such as adaptive cross polarization interference cancelers,and adaptive phased arrays, feedforward amplifiers, and equipment fortesting or monitoring, electronic components and systems.

Thus, an active distributed signal injector for use with high frequencysystems, whose components are interconnected by low loss constantimpedance transmission lines has been disclosed. It is to be understoodthat the described embodiment is merely illustrative of some of the manyspecific embodiments which represent applications of the principles ofthe present invention. Clearly, numerous and other arrangements can bereadily devised by those skilled in the art without departing from thescope of the invention.

What is claimed is:
 1. An active distributed signal injectorcomprising:a first transmission line; a second transmission line; a mainsignal input port coupled to a first end of the first transmission line;a main signal output port coupled to a second end of the firsttransmission line; an injected signal input port coupled to a first endof the second transmission line; a termination coupled to a second endof the second transmission line; a plurality of high impedanceamplifiers bridging between the furst and the second transmission lines;and a plurality of inductors respectively disposed along the first andsecond transmission lines between each of the high impedance amplifiers.2. The active distributed signal injector of claim 1 further comprisinga plurality of transmission lines coupled between selected ones of theinductors of the second transmission line.
 3. The active distributedsignal injector of claim 1 wherein the plurality of high impedanceamplifiers each comprise field effect transistors.
 4. The activedistributed signal injector claim 1 which is implemented as a microwavemonolithic integrated circuit.
 5. The active distributed signal injectorof claim 2 wherein the plurality of high impedance amplifiers eachcomprise field effect transistors.
 6. The active distributed signalinjector claim 2 which is implemented as a microwave monolithicintegrated circuit.
 7. An active distributed signal injectorcomprising:a first transmission line; a second transmission line; a mainsignal input port coupled to a first end of the first transmission line;a main signal output port coupled to a second end of the firsttransmission line; an injected signal input port coupled to a first endof the second transmission line; a second output port coupled to asecond end of the second transmission line that is used to output theinjected signal, a plurality of high impedance amplifiers bridgingbetween the first and is second transmission lines; and a plurality ofinductors respectively disposed along the first and second transmissionlines between each of the high impedance amplifiers.
 8. The activedistributed signal injector of claim 7 further comprising a plurality oftransmission lines coupled between selected ones of the inductors of thesecond transmission line.
 9. The active distributed signal injector ofclaim 7 wherein the plurality of high impedance amplifiers each comprisefield effect transistors.
 10. The active distributed signal injectorclaim 7 which is implemented as a microwave monolithic integratedcircuit.
 11. The active distributed signal injector of claim 8 whereinthe plurality of high impedance amplifiers each comprise field effecttransistors.
 12. The active distributed signal injector claim 8 which isimplemented as a microwave monolithic integrated circuit.